1. Field of the Invention
The present invention relates to a data communication system. More specifically, the present invention relates to such a system which comprises a plurality of controlling computers and devices being controlled which are connected to a common data bus by means of data stations, whereby backup between computers and transmission and reception of a process data between computers and devices are carried out at a high transmission rate.
2. Description of the Prior Art
Of late years electronic information processing systems have progressed toward an on-line and large scale type. A large scale implementation in this context implies not only employment of a larger sized computer but also installation of a number of remote terminals in a wider area. A typical data communication system comprises a plurality of controlling computers and devices being controlled installed in a wider area which are connected to a common data bus through the respective data stations, such that data communication is carried out between the computers or between the computers and devices. Such a system, often referred to as "a data highway system," has become the object of attention of those concerned in iron plants, petrochemical plants and the like, in view of remarkable reduction of a wiring cost in spite of a large size and more complicacy of the plants.
FIG. 1 illustrates a block diagram of a typical prior art data highway system in which the present invention may be advantageously adopted. The system shown comprises a plurality of data stations 101 to 107 interposed in a common loop connected data bus DB in a cascade fashion. Devices such as input devices, output devices, peripheral units and the like are connected to the data stations 103, 104, 105 and 107 and controlling computers 201, 202 and 206 are connected to the data stations 101, 102 and 106, respectively, such that the abovementioned devices are controlled by the abovementioned computers through exchange of information through the data stations and the data bus DB. The device being controlled may be one which is operable in response to a data given thereto from a certain computer through the abovementioned exchange of information, such as a typewriter 301, a cathode ray tube 303, a display 304, or the like, or may be one which serves to provide a data obtained thereat to a certain computer, such as a measuring device 302, a card reader 305 or the like.
Operation of the data highway system shown will be described briefly in the following. Now take an example in which a data is transmitted from the controlling computer 201 to the typewriter 301 or the measuring device 302. The controlling computer 201 serves to provide a data in a form of parallel bit signal. The data, as transmitted, comprises an address of its own data station 101 from which the said data is transmitted, an address of the data station 104 to which the said data is to be transmitted, an address of the device specifying which device should communicate the data (the typewriter 301 or the measuring device 302, for example), a series of character code signals in case where the typewriter 301 is specified, a point address for specifying a measuring point in case where the measuring device 302 is specifed, and other relevant data. The above-mentioned data, as transmitted from the controlling computer 201, is complied in a predetermined format of a series bit signal by the data station 101, and such a compiled or modulated signal is transmitted through the data bus DB, which is typically comprised of a single signal transmission path or two signal transmission paths.
When the above-mentioned modulated signal is transmitted along the data bus DB, all the other data stations 102 to 107 receive the modulated signal and demodulate it therein, whereby the address of the data station 104, as addressed, is compared with its own address stored therein. Only the data station 104 the address of which coincides with its own stored address at the above-mentioned comparison of the addresses is enabled to withdraw the transmitted data from the data bus DB.
The data station 104 serves to check the withdrawn data. If there is no error as a result of the above-mentioned check, the data as withdrawn in the form of a bit series signal is converted to a bit parallel signal and the character code signal or the point address is transmitted to the typewriter 301 or the measuring device 302, as specified by way of the device address, respectively, as the case may be. If any error is detected as a result of the above-mentioned error check, signal is transmitted back to the computer which requests that the data is again transmitted from the computer to the device. This request signal is received by the data station 101 to transmit the same data again to the device. If and when the typewriter 301 is specified by way of the device address, the typewriter 301 receives the transmitted data and typewrites the characters until the typewriting of all the data is completed. If and when the measuring device 302 is specified, the device 302 makes measurement of the measuring points as specified by way of a point address, the measured data of which is transmitted to the data station 104 in the form of bit parallel signal, whence a series bit signal is withdrawn to the data bus DB. More specifically, the data station 104 converts into a series bit signal a data comprising its own data station address, the address of the data station 101 to be addressed, the device address of the measuring device 302, the measured data and the other relevant data, and modulates the bit series signal and transmits the modulated data to the data bus DB. The data station 101 detects its own data station address in the transmitted data and withdraws the data therein. If there is no error as a result of error checking, the data is delivered to the controlling computer 201 after it is converted to a bit parallel signal.
As seen from the foregoing description it is understood that in such a data highway sytem any one of the controlling computers 201, 202 and 203 can control any devices connected to the data stations 103, 104, 105 and 107. Therefore, it can happen that a single particular device being controlled is shared by two or more controlling computers for communication of the data therebetween. Such a problem of "conflict" should be absolutely avoided. For example, assume an instance in which the typewriter 301 is simultaneously controlled by two controlling computers 202 and 206. If such a situation is permitted two sentences from both computers are mixed up, which makes it impossible to read the print out of the typewriter 301. In order to avoid such an inconvenience it is necessary to keep waiting one computer, say 202, until the other computer, say 206, completes transmission of a full data. For this purposes it is desired or rather necessary to provide a hardware for refraining from receiving any request from the other computers, say 206 and/or 201, while one computer 202 is controlling the typewriter 301.